1. Field of the Invention
The present invention relates to a novel thermoelectric conversion material in which a silicon-based polycrystal powder having a crystal structure that includes crystal grains composed of a silicon-rich phase, and a doped element-rich phase in which at least one element is precipitated at the crystal grain boundary, is compounded with an AGaT polycrystal powder with a low thermal conductivity. The present invention also relates to a method for manufacturing a thermoelectric conversion material with a low thermal conductivity, resulting in high thermoelectric conversion efficiency.
2. Description of the Related Art
Thermoelectric conversion elements are devices that are expected to see practical use because of their efficient utilization of the high levels of thermal energy required in industrial fields today. An extremely broad range of applications have been investigated, such as a system for converting waste heat into electrical energy, small and portable electric generators for easily obtaining electricity outdoors, flame sensors for gas equipment, and so forth.
That thermal conductivity can be lowered and the thermoelectric figure of merit raised by the addition of germanium to silicon is known from the reports of J. P. Dismukes et al. (J. Appl. Phys., 35 (1964), 2899) and N.Kh. Abrikosov et al. (Sov. Phys. Semicon., 2 (1969), 1468).
It is also already known that thermal conductivity can be markedly lowered by using a solid solution of hetero elements in a semiconductor based on Sixe2x80x94Ge or InAsxe2x80x94GaAs (xe2x80x9cThermoelectric Conductors and Their Applications,xe2x80x9d by K. Uemura and T. Nishida).
This Sixe2x80x94Ge system is a solid solution in any composition, in the phase diagram of which there is a large temperature differential between the liquidus and solidus, and a problem has been that the silicon and germanium are prone to segregation. Also, the above-mentioned Sixe2x80x94Ge-based materials containing at least 20% germanium have yet to see widespread use, since germanium is a costly raw material.
Meanwhile, G. S. Nolas (Mat. Res. Soc. Symp. Proc., 545 (1999), 435) has reported that clathrate compounds based on Group 4 elements have a complex cage structure, and have an extremely low thermal conductivity due to the xe2x80x9crattling effectxe2x80x9d caused by atoms introduced into the cage-like cluster.
The inventors turned their attention to the fact that silicon, which is widely used in semiconductor devices, has an extremely high Seebeck coefficient, and in particular examined the thermoelectric characteristics of silicon-based materials, and as a result learned that the addition of a small amount (0.001 to 20 at %) of another element to silicon yields a thermoelectric conversion material having a high figure of merit (WO99/22410).
The thermal conductivity of the above-mentioned silicon-based materials can be lowered by using various doping elements, and the Seebeck coefficient at a specific carrier concentration will be equivalent to or better than that of conventional Sixe2x80x94Ge and Fexe2x80x94Si compounds. Such a material exhibits a good figure of merit as a thermoelectric conversion material, allowing it to contribute to higher performance.
In general, thermal conductivity (K) is the sum of conduction (Kel) by carriers (free electrons) and conduction (Kph) by phonons. Since conduction by phonons is dominant in the semiconductor region (carrier concentration  less than 1021 M/m3), the scattering of phonons needs to increase in order to lower the thermal conductivity.
Adding impurity elements to the silicon is effective to this end, but there is a limit to the amount of Group 2 and 3 or Group 5 and 6 elements, transition metal elements, and rare earth elements that can be added, and the thermal conductivity that could be achieved at room temperature with such a method was only about 10 W/mK at best.
The value of this thermal conductivity (K) is still about one order of magnitude larger than that of other thermoelectric materials, and thermal conductivity needs to be further lowered in order to further increase the thermoelectric figure of merit (Z).
It is an object of the present invention to provide a thermoelectric conversion material, and a method for manufacturing this material, with which the thermal conductivity of a silicon-based thermoelectric conversion material can be greatly lowered without lowering the Seebeck coefficient or electrical conductivity of the material, which affords a marked increase in the thermoelectric figure of merit.
As a result of various investigations into the compounding of a silicon-based thermoelectric conversion material (silicon-based polycrystals) with other materials in an effort to lower the thermal conductivity of said silicon-based material, the inventors discovered that the thermal conductivity can be lowered considerably without greatly sacrificing the Seebeck coefficient by compounding this silicon-based material with a material that has a lower thermal conductivity than silicon-based polycrystals and that also has lower electrical resistivity.
The inventors perfected the present invention upon discovering that a thermoelectric conversion material with a markedly lower thermal conductivity can be obtained without sacrificing a high Seebeck coefficient and low electrical resistivity by mixing a silicon-based polycrystal powder composed of a p- or n-type semiconductor in which any of various elements have been added to a silicon-based material containing germanium, for example, in an amount of 0.05 to 10 at % to adjust the carrier concentration to between 1019 and 1021 M/m3, at which the Seebeck coefficient is high, with a separately prepared clathrate compound having low thermal conductivity and electrical resistivity, and then compounding this mixture by hot pressing.
Specifically, the thermoelectric conversion material pertaining to the present invention is a composite of silicon-based polycrystals and AGaT polycrystals, wherein these silicon-based polycrystals have a crystal structure containing an added element or a combination of added elements in an amount of 0.001 to 30 at % and include crystal grains made up of 80 at % silicon, and a grain boundary phase where at least one type of said added element is precipitated at the boundary of the crystal grains, and these AGaT polycrystals contain at least one Group II-A element selected from among Be, Mg, Ca, Sr, and Ba as A, and at least one Group IV-A element selected from among C, Si, Ge, Sn, and Pb as T.
The inventors also propose thermoelectric conversion materials having the structures of the following 1) to 5) as variations on the thermoelectric conversion material with the above structure.
1) Wherein the thermoelectric conversion material has a structure in which the particles of the AGaT polycrystals are disposed around the particles of the silicon-based polycrystals;
2) Wherein the added element in the silicon-based polycrystals include 0.1 to 10 at % of at least one of added element that does not generate carriers and 0.002 to 20 at % of at least one type of added element that does generate carriers, and the added element that does generate carriers includes at least one element selected from among Ge, C, and Sn;
3) Wherein the AGaT polycrystals have a compositional formula of AXGaYTZ, where X=8, Y=14 to 17, and Z=28 to 32;
4) Wherein the silicon-based polycrystals are of the same conduction type as the AGaT polycrystals; and
5) Wherein the mixing weight ratio of the silicon-based polycrystals and the AGaT polycrystals is 60:40 to 90:10.
The inventors further propose a method for manufacturing a thermoelectric conversion material, characterized by comprising the steps of mixing (mixing and pulverizing) a silicon-based polycrystal powder and an AGaT polycrystal powder, and compounding the mixed powder by hot compression molding, such as hot pressing or discharge plasma sintering, or more specifically, creating a composite structure in which particles of clathrate compound polycrystals are disposed around the particles of the silicon-based polycrystals.
With the present invention, the thermal conductivity of silicon-based polycrystals having a high thermoelectric figure of merit can be greatly lowered without lowering the Seebeck coefficient or electrical conductivity of this thermoelectric conversion material, which affords a marked increase in the thermoelectric figure of merit.